For various applications, variable displacement hydraulic apparatus such as motors and pumps have demonstrated greater system efficiencies than their fixed displacement counterparts. Typically, a hydraulic system will operate at a variety of loads and speeds and the ability to change flow on command reduces input shaft horsepower requirements and heat generated therein. For such purposes, the variable displacement hydraulic apparatus can be configured in such a manner as to vary the port plate angle in relation to the cylinder block.
To vary the output of a hydraulic apparatus, it is also well known to use a variable swash plate which "wobbles" back and forth to vary the displacement of the pistons therein. When a wobbler is used, the swash plate against which the piston shoes slide is moved about an axis of rotation to change the piston displacement. At one extreme, if the swash plate is precisely perpendicular to the central axis of rotation, no change in displacement would occur as the pistons rotate around the motor centerline.
Another way of regulating the output of a hydraulic apparatus is to rotate the port plate about the centerline of the hydraulic apparatus which causes the inlet and outlet kidneys to move with respect to the wobbler or cam to thereby affect the operation of the apparatus. It will be understood that this technique can be used with either a variable or fixed swash plate as long as the swash plate is always at some non-perpendicular angle relative to the central axis of rotation.
With reference to the second technique wherein the port plate is rotated about its centerline which is also the centerline of the hydraulic apparatus, it has been known to utilize a hydraulic servo valve and piston arrangement to cause the rotational indexing of the port plate. As will be appreciated, the hydraulic servo valve controls the oil ported to the piston arrangement and moves the port plate by some predetermined amount about its centerline relative to the position of the hydraulic apparatus.
As a result, hydraulic apparatus have traditionally utilized hydraulic servo valves to vary displacement. Electrical inputs to the valve control flow to the control piston, thereby changing the angle of the plate, thus changing unit displacement. Unfortunately, such hydraulic servo valves have a serious drawback in that quiescent fluid is oftentimes leaked.
As for other approaches, control of a port plate has been proposed by mechanical and hydromechanical devices of various types. For instance, Budzich U.S. Pat. No. 3,190,232 attempts to control a port plate by means of a piston driven ball and Firth et al. U.S. Pat. No. 3,117,529 attempts to achieve this objective by means of a o rack and pinion arrangement. However, both of these methods of controlling a port plate are less than satisfactory since they would provide unbalanced forces.
The present invention is directed to overcoming the foregoing problems and achieving the resulting objectives.